Image forming apparatuses, such as copying machines using well known electro-photographic methods, charge electricity on their image carrier and expose it to form an electrostatic latent image. The electrostatic latent image, formed on the image carrier, is changed to a toner image by the developing device, and said toner image is transferred onto a recording sheet by a transfer device. After that, said toner image is heated to be fixed by a fixing device, to become a permanent image on the recording sheet.
The developing device uses a dual-component developer, including toner and carrier, and develops the electrostatic latent image formed on the image carrier, and generates a visible image.
After the developing process has been completed on a developing area the dual-component developer, in which the toner has been consumed, is recovered. Subsequently, the recovered developer is mixed with supplied toner, and said mixed developer is agitated to be used again for further image development. The developer, which is used in the developing device exhibiting the above recovering structure, is required to maintain a specific toner density, electrification amount, and additives, respectively at predetermined values, to obtain stable toner images. The toner density is determined by the toner, consumed in the developing operation, and the distribution of replenished toner. The electrification amount is determined by the frictional electrification, generated when the carrier and the toner are mixed. The developing device adequately conducts the agitation of dual-component developer, formed of a toner and carrier, so that the distribution of toner density is effectively uniformized, and the electrification amount to be applied to the toner can be saturated, which results in a stabilization of the toner image.
When toner consumption for a single print is relatively small, the staying time of the toner in the developing device becomes longer. The additives tend to separate from or become buried in the surface of the toner, and the electrification amount increases due to the agitation, which results in deterioration of the image quality. Various technologies are well known to maintain the image quality.
In order to overcome the deterioration of image quality, when toner consumption for a single print is relatively small, there is a well known method in which the toner is consumed at an area which is out of an image forming area, so that the staying time of the toner in the developing device is reduced. However, the toner is consumed needlessly, which adversely results in the price for a single print.
According to Unexamined Japanese Patent Application 2003-255,682, an external additive supplying section is prepared to supply external additives. When the toner supplying section supplies relatively small amounts of toner, an external additive supplying section is controlled to supply a large amount of external additive, so that fogged images or flying toner particles are effectively controlled.
According to Unexamined Japanese Patent Application 2007-310,191, a toner accommodating section and an additive accommodating section are prepared in the toner supplying section. When the accommodated toner is reduced, the additive is supplied greater from the additive accommodating section, so that filming of the photoconductive body is effectively prevented.
According to Unexamined Japanese Patent Application 2007-163,592, due to the long operation of the developing device, when the remaining toner is excessively deteriorated, toner particles, exhibiting the different diameters from that of initially supplied toner particles, are supplied to the developing device, so that the image quantity can be stabilized.
According to Unexamined Japanese Patent Application 2008-262,185, toner to be supplied, exhibiting different diameters, can be set. Based on the diameter of the toner particles to be supplied, exposure energy, which serves as an image forming condition, is controlled so that the image quantity can be stabilized.
According to Unexamined Japanese Patent Application 2010-19,897, two kinds of toners, exhibiting different developing power, are used. When an image having a large image area is to be outputted, because the toner in the developing device increases the developing power, a large amount of toner having low developing power is supplied. When an image having a small image area is to be outputted, a large amount of toner having high developing power is supplied, so that the variation of developing density can be controlled.
As detailed above, deterioration of developer in the developing device is problematic. Specifically, when the amount of toner consumption per single print is low, the toner particles, which did not transfer from the developer carrier to the image carrier, are recovered and reused, whereby the time of the toner in the developing device becomes longer. Accordingly, additives tend to separate from or are buried in the surface of the toner particle, which leaves much mom for improvement.
To apply additives onto the surface of toner particles is effective for flowing capability and transferring capability. However, when the developer is used, which includes toner particles from which the additives have been separated, or on the surface of which the additives have been buried, the additives cannot function normally, that is, the flowing capability and the transferring capability become deteriorated. Said deterioration of the flowing capability adversely affects regulation of the surface of the developer, formed like brushes, which results in the generation of a rough image. This becomes conspicuous by the reverse phenomenon, in which the brushes strongly touch the image carrier. Further, deterioration of the transferring capability also results in the generation of the rough image.